Biological studies in the vicinity of a shallow-sea tidal mixing front IV. Seasonal and spatial distribution of urea and its uptake by phytoplankton

Abstract

On six cruises in 1980 the vertical and horizontal distributions of urea concentration and uptake rates were determined in the vicinity of a shallow-sea tidal mixing front in the western Irish Sea. Urea concentrations, while patchy, were similar throughout the year and showed no significant patterns of distribution and no relation to stratification of the water column. Urea uptake rates, on the other land, showed a consistent and strong relationship to stratification, both vertically and horizontally, fastest rates being found in the less dense water on the stratified side of the front and above the pycnocline. Large differences between waters above and below the pycnocline were found during months of strong stratification. Similar differences occurred between the stratified and mixed surface waters on each side of the front. From relatively low urea uptake rates in March, when stratification was weak, extremely high rates were observed in June and thereafter decreased attaining another minimum at the end of September when stratification was weakening. Urea uptake indices (uptake per unit of chlorophyll a ) were also highest in the surface stratified waters and followed a similar vertical, horizontal and seasonal distribution pattern as that of urea uptake rates. The seemingly unchanging urea concentrations throughout the year and its extremely fast uptake by micro-organisms indicate a rapid flux of this nitrogenous com pound in the surface of the stratified waters. The possible routes of urea regeneration are discussed. Budget calculation indicate that urea was an important source of nitrogen for phytoplankton in the surface stratified waters when oxidized forms of nitrogen such as nitrate were depleted and that the rapid flux of reduced nitrogen in the form of urea may be a major factor in sustaining high productivity associated with the frontal system.